Hydrogen has many current industrial uses and potential future uses involving the supply of energy in fuel cells. For example, there presently exists a continuing need for hydrogen to treat high sulfur content crude oil in the production of fuels. In the future, hydrogen may be used as an automotive fuel or more generally, in the generation of electricity.
Hydrogen is currently produced in steam methane reforming installations. In such installation, a hydrocarbon containing feed, typically natural gas, is combined with steam and then introduced into reformer tubes located within a fired furnace of the reformer. The reformer tubes contain a catalyst to catalyze the well known, endothermic steam methane reforming reaction in which methane and steam are reacted to form carbon monoxide and hydrogen. In addition, an exothermic water gas shift reaction occurs in which carbon monoxide and steam are reacted to form carbon dioxide and additional hydrogen. Typically, the hydrocarbon containing feed is natural gas and part of such feed is combined with steam and introduced into the reformer tubes and another part of the feed is fed to burners firing into the furnace section to support the steam methane reforming reaction. In typical steam methane reformers, the steam to carbon molar ratio is set at about 2.8.
The heated product stream of the steam methane reforming reaction is cooled and subjected to a high temperature shift reaction to react the carbon monoxide with residual steam to produce additional hydrogen. The resulting shifted stream is then introduced into a pressure swing adsorption unit in which the hydrogen is separated to form a product stream and a stream of tail gas is produced that can be introduced into the burners to help fire the furnace section of the steam methane reformer.
Steam methane reformers also have a convective section connected to the furnace section in which flue gas is routed to heat boiler feed water and to produce steam. The boiler feed water after deaeration and heating to near its boiling temperature is then introduced into a steam drum. Water from the steam drum is partially vaporized in the boiler and returned to the steam drum as low quality steam. Steam from the steam drum is introduced into a superheater in the convective section to form superheated steam. The superheated steam is combined with the hydrocarbon feed to produce the reactant stream for the steam methane reformer and part of the superheated steam can be advantageously exported at a profit. The flue gas is discharged from the convective section through a stack.
Steam methane reformers can utilize a variety of feed stocks, for instance, refinery off-gases, natural gas, butane, light naphtha and naphtha. All of these are hydrocarbon containing feeds. In Broadhurst et al., “Effects of Hydrocarbon Feed Type on Operating Costs and Environmental Impact on a Steam Reforming Based Hydrogen Plant”, AICHE (2005), various feeds were tested against one another by simulation at a steam to carbon ratio of 3.0 and a reformer exit temperature of 880° C. The feed rates were allowed to vary so that a target hydrogen output of 100 MMSCFD was produced. From the simulations, it was concluded that the environmental impact increases in terms of higher carbon dioxide emissions and lower process efficiency as the feed stock becomes heavier. This being said, it was surmised in this reference that the selection of the feed stock will be dominated by feed stock/fuel costs.
Another method of producing hydrogen is gasification in which a carbonaceous material such as coal, petroleum or biomass is converted into a synthesis gas that contains hydrogen and carbon monoxide. The carbonaceous material is reacted at high temperatures with oxygen addition within a gasifier to produce the synthesis gas. For example, in one type of gasifier that is used in the gasification of coal, the coal is pulverized and fed into the gasifier. Other types of gasifiers utilize a coal slurry. Within the gasifier, the coal is heated and volatiles are released creating a char. The volatile product and some of the char is reacted with the oxygen to form carbon dioxide and carbon monoxide. The char also reacts with the carbon dioxide and steam to produce the carbon monoxide and hydrogen. In addition, carbon monoxide and steam also react in water gas shift reactions to produce carbon dioxide and additional hydrogen. The resulting hydrogen and carbon monoxide containing synthesis gas can be processed and hydrogen can be separated from the synthesis gas by pressure swing adsorption.
In Gray et al. “Polygeneration of SNG, Hydrogen, Power, and Carbon Dioxide from Texas Lignite”, Mitretek Systems (2004), the gasification of lignite is discussed in connection with the production of electric power, hydrogen, synthetic natural gas and carbon dioxide. In this reference, the purpose of such gasification is to allow the electric power to be sold to the grid, the hydrogen to be sold to a pipeline for use in oil refining operations and the synthetic natural gas to be sold as a natural gas supplement or to replace natural gas in steam methane reforming operations, thereby to provide hydrogen for the refining operations. The carbon dioxide that is generated by the gasification can be sequestered or used for enhanced oil recovery.
In one plant configuration shown in Gray, the lignite is gasified to produce a synthesis gas. The synthesis gas is subjected to water gas shift reactions to increase the hydrogen. After removal of mercury, sulfur and carbon dioxide, the shifted stream is then passed into a sulfur guard bed and then into a methanation unit to produce synthetic natural gas. Sulfur can be extracted in a Claus unit for sulfur recovery. The carbon dioxide can be compressed to 2,000 psi and fed to a pipeline. To protect the methanation catalyst, the purified and shifted syngas is sent to a sulfur polishing reactor to remove the last traces of hydrogen sulphide before being sent to the methanation reactor. In the methanation reactor the carbon dioxide and hydrogen are reacted to produce methane. The resulting synthetic natural gas is compressed for delivery to a natural gas pipeline. Some of the synthesis gas can be sent to a gas turbine where electric power is generated. The hot effluent from the gas turbine can be used in a heat recovery steam generator to generate high pressure steam that is used in a steam turbine to generate additional electrical power.
In another plant design that is shown in Gray, after carbon dioxide removal, the purified and shifted syngas is sent to a polymer membrane separation system followed by a pressure swing adsorption unit where hydrogen is removed. The remaining synthesis gas is then sent to a gas turbine for electric power generation. The heated effluent from the turbine is then sent to a heat recovery steam generator to generate high pressure steam for use in a steam turbine to generate additional power.
As is apparent from the above description of the prior art, steam methane reformers can utilize a variety of feeds, natural gas, refinery off-gas, synthetic natural gas and mixtures of synthetic natural gas and natural gas. Obviously, the amount of synthetic natural gas utilized as a feed to a steam methane reformer will depend upon such economic factors as the price of natural gas.
As will be discussed, the present invention provides a closer integration between a gasifier and a steam methane reformer then has been contemplated in the prior art. Among other advantages, a method in accordance with the present invention allows the hydrogen to be produced in a steam methane reformer with the use of less natural gas or synthetic natural gas than is possible in the prior art. Thus, the present invention permits use of natural gas or production of synthetic natural gas to be more widely varied in response to external economic conditions than is possible in the prior art. As will be discussed, yet further advantages of the present invention concern the possibility of simplifying the construction of the gasification facility and providing for a more reliable hydrogen supply than is currently possible with the use of a gasification facility alone.